A grain-oriented electrical steel sheet is mainly utilized as an iron core material of a transformer or the like, and is required to have excellent magnetization characteristics, in particular low iron loss. In this regard, it is important to highly accord secondary recrystallized grains of the steel sheet with the (110)[001] orientation (or so-called Goss orientation) and to minimize impurities and precipitates present in steel of finished products. However, there are limitations in controlling crystal orientation and reducing impurities and precipitates in terms of balancing with manufacturing cost, and so on. Accordingly, there has been developed a technique of introducing non-uniformity into a surface of a steel sheet by physical means to subdivide width of magnetic domains to reduce iron loss, i.e. a magnetic domain refining technique.
For example, PTL 1 (JPS57002252B) and PTL2 (JPH0672266B) disclose a technique of irradiating a surface of a finished product steel sheet with a laser beam or an electron beam in a direction substantially orthogonal to the rolling direction with intervals of several millimeters, to introduce linear high-dislocation density regions (strains) into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss.
While the strains subdivide width of magnetic domains to reduce iron loss, they cause local deformation in steel sheets. Generally, since strains for magnetic domain refining are introduced in one side of the steel sheet, a deflection where the strain-introduced surface becomes the inner side inevitably occurs. Conventionally, this deflection was considered to deteriorate characteristics of the grain-oriented electrical steel sheet such as iron loss properties and magnetostrictive properties, and techniques for limiting the area of deflection have been disclosed. For example, PTL3 (JP2012052228A) discloses a grain-oriented electrical steel sheet with reduced iron loss, obtained by satisfying a predetermined relation between the tension-applying insulating coating and the tension applied to the steel sheet surface before strain-introducing treatment, and limiting the magnitude of deflection of the steel sheet per length of 280 mm of a strain-introduced surface side after strain-introducing treatment, to 1 mm or more and 10 mm or less, in particular 3 mm or more and 8 mm or less.
Here, the magnitude of deflection if the steel sheet depends on irradiation conditions such as laser beam or electron beam at the time of introducing strains. Beam power, beam scanning rate, beam spot shape, and irradiation interval are conditions which have a particularly great influence.